This invention relates generally to a method, apparatus, and system for determining a location of a frequency synchronization signal. More particularly, this invention relates to a method, apparatus, and system for determining a location of a frequency synchronization signal among data transmitted from a transmitter and received by a receiver in a communication system.
In any communication system, it is important for a receiver to be synchronized with a transmitter so that messages can be successfully exchanged between the transmitter and the receiver. In a radio communication system, in particular, it is important that a receiver be tuned to the frequency of the transmitter for optimal reception.
In a typical radio communication system, remote stations communicate with one or more base stations via a radio air interface. Various approaches have been employed to prevent transmissions between the various base stations and remote stations from interfering with each other.
In some radio communication systems, neighboring base stations are each assigned a different carrier frequency with which to communicate with remote stations so that transmissions from one base station do not interfere with transmissions from a neighboring base station. In addition to such a Frequency Division Multiple Access (FDMA) technique, Time Division Multiple Access (TDMA) has been employed. In systems using TDMA, a base station may allocate a particular time slot or slots within a frame on a carrier to each remote station. Some remote stations can use the same carrier frequency but different time slots to communicate with the base station.
In other radio communication systems, the Code Division Multiple Access (CDMA) method has been employed. According to the CDMA method, each remote station is assigned a particular digital code word(s) that is orthogonal to code words assigned to other stations. Neighboring base stations can exchange messages with remote stations using the same frequency but different digital orthogonal code words to indicate which remote station the messages are designated for.
Whether a radio communication system employs FDMA, TDMA, CDMA, a combination of these approaches, or some other approach, it is important for a remote station to be time and frequency synchronized to the base station serving the area from which it desires to communicate. In other words, the frequency reference of the remote station must be tuned to the carrier frequency of the base station, and the time reference of the remote station must be synchronized to the time reference of the base station. A periodic synchronization signal is typically transmitted from the base station to the remote station for this purpose.
In a system complying with the European Global System for Mobile Communication (GSM) standard, information is transmitted from the base station to a remote station by modulating the carrier of the base station with, e.g., a Normal Burst (NB) of data. To synchronize the mobile station to the base station, the carrier of the base station is also modulated from time to time with a Frequency Correction Burst (FCB) and a Synchronization Burst (SB) to form a frequency synchronization signal.
The carrier of the base station is typically modulated with the FCB using Gaussian Minimum Shift Keying (GMSK). In a GSM system, a FCB is a sequence of 148 symbols, each symbol a zero, that transforms into a pure sinusoidal signal after modulation. The frequency of the resulting frequency synchronization signal is thus equal to 1/4T Hz, where T represents a symbol duration. T is typically 48/13 microseconds (xcexcs), so that the frequency synchronization signal has a frequency of approximately 67.7 KHz. The FCB is repeated every tenth frame for the first four times, and then for the fifth time, the FCB is repeated on the eleventh frame. This frame sequence is then repeated indefinitely, to maintain synchronization between the remote station and the base station.
From the information in the FCB, the remote station is able to roughly synchronize itself with the time slot(s) allocated to it. This rough time synchronization is then sufficient to locate the SB, which is typically located eight bursts after the FCB, and to decode the information it carries. The information obtained by decoding the SB is then used to finely tune the frequency reference of the remote station to the carrier frequency of the base station and to adjust the remote station""s time reference to the time slot(s) allocated to it by the base station.
In systems employing CDMA, each base station transmits a frequency synchronization signal in the form of, for example, a pilot sequence on each of the frequencies assigned to that particular base station as well as possibly on some or all of the frequencies that are not assigned to that particular base station. If the frequency has been assigned to the base station, the corresponding pilot sequence may be transmitted with slightly more power than the other frequencies used by the base station. Each remote station receiving the carrier modulated by the pilot sequence demodulates the signal. As a result, each remote station can receive signals designated for it and simultaneously measure the signal strengths of neighboring base stations using different pilots or carriers. This information is used by the remote station to determine which received pilot sequence has the strongest signal strength, and the frequency reference of the remote station is adjusted to the appropriate carrier frequency, accordingly.
Any frequency difference between the frequency reference of the remote station and the carrier frequency of the base station is readily detected in the demodulated frequency synchronization signal. For example, in systems complying with the GSM standard, the difference between the frequency of the modulated frequency synchronization signal, which is known to be 67.7 KHz, and the frequency of the received frequency synchronization signal, demodulated to the baseband, is a direct measure of the error in the frequency reference of the remote station. In systems employing CDMA, the difference between the known frequency of the strongest transmitted pilot sequence and the frequency of the demodulated pilot sequence is used by the remote station as a measure of the error in the frequency reference of the remote station.
In order to synchronize a remote station to a base station, it is therefore important to accurately detect the frequency synchronization signal transmitted from the base station and to estimate the frequency difference between the remote station""s frequency reference and the carrier frequency of the base station.
Many techniques have been proposed for detecting the frequency synchronization signal. One such technique is disclosed in a commonly assigned U.S. patent application Ser. No. 09/026,724 entitled xe2x80x9cMethod and Apparatus for Detecting a Frequency Synchronization Signalxe2x80x9d, filed on Feb. 20, 1998 in the names of Roozbeh Atarius et al., now U.S. Pat. No. 6,226,336 and herein incorporated by reference. This detection method uses the similarities between the in-phase and quadrature-phase components of received signals to detect a frequency synchronization signal.
Many techniques have also been proposed for estimating the frequency difference between the remote station""s frequency reference and the carrier frequency of the base station. One such technique is disclosed in commonly assigned U.S. patent application Ser. No. 08/971,666, filed on Nov. 17, 1997, now U.S. Pat. No. 6,104,767 and herein incorporated by reference. This estimation method uses the phase differences between successive samples of a detected frequency synchronization signal to estimate the frequency offset.
To accurately estimate the frequency offset and thus tune the remote station to the carrier frequency of the base station, it is important to know the actual location of the frequency synchronization signal among data received by the remote station, e.g., where the FCB occurs in a frame. Otherwise, the signal used in the frequency offset estimation may not correspond entirely to the frequency synchronization signal, which may result in less than optimal tuning. Thus, there is a need to determine the location of a detected frequency synchronization signal among data received by a receiver.
It is therefore an object of the present invention to determine the location of a frequency synchronization signal. According to an exemplary embodiment of the present invention, this and other objects are met by a method, apparatus, and system for determining the location of the frequency synchronization signal among data transmitted from a transmitter and received by a receiver. A peak value representing a signal detected by the receiver is calculated, a frequency offset between the frequency reference of the receiver and the carrier frequency of the transmitter is estimated, and a quality factor indicating the accuracy of the estimated frequency offset is estimated.
The location of the frequency synchronization signal is determined by comparing the calculated peak value and the estimated quality factor with predetermined peak and quality thresholds, e.g., determining whether the peak value is greater than or equal to a peak threshold and whether the quality factor is less than or equal to a quality threshold. When both of the predetermined threshold conditions are met, the peak value, frequency offset, and quality factor are stored. When either of the threshold conditions is not met, the location of the frequency synchronization signal corresponds to the location of a stored maximum peak value and a stored minimum quality factor.